Tunable Dewatering Behavior of Montmorillonite Suspension by Adjusting Solution pH and Electrolyte Concentration

Abstract

Montmorillonite is always a troublemaker for the dewatering in coal processing since its existence can decrease the rates of sedimentation and filtration of coal slurry. To eliminate the adverse effect of montmorillonite, adjusting the slurry pH and adding electrolytes are always the key methods. However, the underlying mechanism still needs to be further studied. The dewatering of Na-montmorillonite (Na-Mt) suspensions has been studied as a function of NaCl concentration (10−3, 10−2, and 10−1 M) at different pH values (6.0, 7.7, 8.1, 9.2). The point of zero charge of edge surface of Na-Mt (pHPZC,edge) appeared at the pH value of 6.8. The sedimentation and rheology experiments described the coagulation and flow behaviors of Na-Mt suspensions, respectively. The Na-Mt suspension coagulated spontaneously at low salt concentration with the pH ~ 6.0. For the pH > pHPZC,edge, the height of the sediment bed reduced and apparent viscosity increased with the increase of the electrolyte concentration. The filtration properties were evaluated on the basis of Darcy’s law. The obtained result clearly demonstrated that the filtration rate was accelerated with the increase of pH and electrolyte concentration. The modes of particle association and its effect on filtration performance were discussed. Moreover, a comparison with related results from the literature was performed. At pH ~ 6 and low electrolyte concentration, the positively charged Edge surfaces and negatively charged Face surfaces coagulate rapidly to form a sealed structure by electrostatic attraction. Furthermore, inside this sealed structure, the water molecules cannot be removed in the filtration process easily. However, by increasing the electrolyte concentration at pH > pHPZC,edge, the gradually formed Face/Face structure increases the filtration rate sharply because of the inhibiting effect of the electric double layer (EDL) and the osmotic expansion. Therefore, adjusting solution conditions of the aqueous suspension to tune the particle coagulation behavior is one of the effective methods to solve the problem of montmorillonite dewatering.